The present invention relates generally to a single-phase brushless motor, and particularly to such a motor which includes a single position detector for detecting a position of a rotary magnet relative to a stator armature and a device for controlling driving currents in each of the coils of the stator armature in response to the output of the single position detector.
In a motor driven by a single-phase half-wave rectified current produced in response to the output of the single position detector, so-called dead points are present where the electromagnetic driving torque decreases to zero, at rotary angles (electrical angles) of 0.degree. and 180.degree., as indicated in FIG. 1A, where the motor is incapable of self starting. To obviate this difficulty, a motor construction as shown in FIG. 2 has been employed in which the shape of the outer circumference of a core 3, wound with a coil 2 of the stator armature 1, is formed so that an armature gap 5 between the outer circumference of the core and rotary magnet 4 varies, to thus vary the distribution of the magnetic energy between the centers of the magnetic poles of the magnet 4 and the core, thereby shifting the peak of the magnetic torque curve (b) by 90.degree. relative to the electromagnetic driving torque curve (a), as shown in FIG. 1A. Thus, as seen in FIG. 1B, the driving torque composed of the electromagnetic driving torque (a) and magnetic torque (b) never falls to zero. A motor operating on these principles is disclosed in Japanese Published Patent Application No. 50,411/74.
In the conventional motor as shown above, since the shape of the outer circumference of the core 3 is not completely circular, assembly jigs used for laminating the core, for coating the core with an insulating material and for winding the coil on the core, and a casting mold used for pressing the core must be complex, this results in high manufacturing cost. In addition, since the amount of magnetic energy generated is determined by the shape of the outer circumference of the core, it is necessary to vary the shape of the core to adjust the amount of magnetic energy, making it impossible to use a single core in a wide variety of applications.
Moreover, considering that only the positive part of the magnetic torque is actively used for driving the rotor, since the negative part operates to oppose the rotation of the rotor, because the magnitude of the negative part of the magnetic torque is substantially equal to that of the positive part, as shown in FIG. 1A, large ripples in the output torque occur in the conventional single-phase brushless motor, as shown in FIG. 1B.